Radio transmitters amplify input signals. It is desired that the gain of such transmitters be linear for the entire range of input signals. A common architecture of recently proposed baseband power amplifier linearizers includes a digital nonlinear gain block, usually called a predistortion block, inserted in the transmitter chain prior to upconversion stages. The predistortion block may be continuously adapted to approximate the inverse nonlinear complex gain of the following transmitter stages up to the power amplifier.
In a conventional linearizer, the non-linearity of the transmitter and power amplifier is corrected by applying an inverse non-linear gain characteristic to the input signal. The non-linear gain characteristic can be determined using a linear receiver and adaptive predistortion controllers. A delayed input signal is compared to an observed sample of the output signal. The amount of delay applied to the input signal is related to the delay of the transmission and observation systems.
In some amplifiers, an amplifier bias may be modified as a function of the envelope of an RF signal. The bias signal is manually synchronized to match up with the input signal. A fixed delay may be provided by a desired length of cable. In some cases, it may be difficult to use a proper length of cable, or the power amplifier may be changed, resulting in a new cable length being needed to obtain the proper delay for the new amplifier. While dynamic biasing of the power amplifier (PA) is known to increase power efficiency, bias signal misalignment with the transmission signal causes interference with the synchronization of the delayed input signal and observed signal resulting in undesired emissions in the output signal.